Melt-Blowing Processes
In one existing melt-blowing process, molten resin is extruded through a row of linearly disposed holes drilled in linear array on about 1 to 2 mm centers into a flat surface about 1 to 2 mm wide, with the surface being located as shown in FIG. 1 at the apex of a member having a triangular cross section, and with the angles at the apex being about 45.degree. to 60.degree. to the center line. Surrounding the apex 11 as shown in FIG. 1 are two slots 12, 13, one on each side, through which is delivered heated air, which attenuate the molten resin extruded through the holes, thereby forming a stream of fibers. The fibers are collected on one side of a moving screen which is separated from the nozzle tips by about 10 or more cm, with the other side of the screen being connected to a suction blower. In operation, most of the fibers are collected on the screen to form a low density web with a rough surface; however, a significant proportion of the fibers escape into the surroundings, and a suction hood is provided from which they are collected and sent to waste.
The collected web is quite weak, with tensile strengths well below about 1.5 kg/cm.sup.2. The fibers have a wide size distribution, with the largest fibers being ten or more times larger than the smallest, and the average fiber diameter is about five to seven or more times the smallest fiber diameter. Many of the fibers are twinned, with a twin being defined as two parallel fibers adhering to each other along a length of 20 or more times their average diameters, while others are roped, i.e., consist of two or more fibers twisted about each other in a form resembling a rope. Roped fibers behave in practice, for example in filtration, much like a single fiber of diameter about equal to that of the rope. Both twinning and roping cause the collected web to have a high pressure drop and low filtration efficiency. Fibers bonded in a twin are less efficient in a filter than two separate fibers. Shot--i.e., small pellets of unfiberized resin interspersed in the web--are also a problem. Typically, the collected web produced as described above reflects a compromise between making shot and rope. This web also tends to have a rough, rather fuzzy surface, that is undesirable for many applications, for example, for use in disposable clothing.
The system for producing webs described above is inefficient by virtue of its geometry. When the two air streams converge, a portion of the energy required to form fibers of the resin is dissipated in proportion to the component of their velocities perpendicular to the center line of the apparatus. A further inefficiency is the rectangular shape of the air stream which acts on each nozzle; if for example 0.5 mm diameter holes are located on 2 mm centers, a rectangular air stream 2 mm wide acts on each 0.5 mm diameter resin passageway. Because the liquid stream is circular, that portion of the air issuing from a corner of the rectangle farthest from the resin nozzle is relatively ineffective, generating a high degree of turbulence with a relatively small contribution to fiber formation. As a result of these inefficiencies, the cost of energy to compress and heat the air in this process is much larger than it would be if each resin nozzle were to receive its own supply of air through a circular annulus. Due to the high volume of air required to fiberize a given weight of resin, the distance from the resin nozzle outlet to the fiber collection surface in usual practice exceeds about 10 to 13 cm, and this relatively long passage through very turbulent air causes the undesirable roping and twinning in the fibers of the collected web. Attempting to operate at much less than 10 cm makes collection on the vacuum screen difficult, unless the fibers are so hot as to be semi-molten, which produces a near to solid product which is inefficient as a filter. In a basic deficiency of this system, the molten resin is disrupted external to the fiberizing die and is simultaneously attenuated to form fibers; there is no clear delineation between disruption and fiber formation, and as a result control of fiber formation is poor.
Another existing system producing attenuated fibers provides for disrupting the molten resin externally to the die. The fibers produced are collected in a randomly oriented heterogenous intertwined arrangement on the mandrel. By the time the fibers reach the mandrel, they are either already broken up or disrupted into discontinuous lengths, or they are still attached to the orifice from which they are spun by a portion which is molten.
The present invention employs self-contained individual fiberizing nozzles comprising an annular air passage. These fiberizing nozzles are capable of making fibrous sheet media with average fiber diameters of about 2 .mu.m or less, and can be operated at a die-to-collector distance (hereinafter DCD) in the range of about 2.5 to about 11.0 cm, e.g., in the range of about 2.8 to about 9.0 cm, and make product with controlled orientation of the fibers.
Such a fiberizing nozzle is depicted in FIG. 2, wherein the fiberizing nozzle 21 contains a capillary 22 through which the molten resin is pumped and a circular annulus 23 through which hot air is delivered. The pumped resin exits the capillary 22 into the resin disruption zone 24 and then into the nozzle channel 25 where the resin, now fragmented into tiny droplets, is carried in the air stream out of the nozzle tip 26, beyond which the individual tiny droplets are elongated into fibers.
Because the air supply is used more efficiently and is correspondingly less in proportion to the weight of the product web, the fiberized product of the present invention can be collected as a web by impinging it on a solid collecting surface, as opposed to the vacuum backed screen of the inefficient apparatus summarized earlier. In another marked improvement on the prior art, the DCD (distance between the nozzle tip 26 in FIG. 2 and the target collecting surface) may be shortened to under about 5.5 cm, illustratively, about 2.5 to about 5.5 cm, e.g., about 2.8 to about 5.5 cm, i.e., about one half or less than used for other systems, thereby reducing the width of the fiber stream and further improving fiber collection efficiency.
The fibers of the instant invention are formed within the fiberizer nozzle, and can be seen by direct observation through a microscope to be fully formed and not in contact with the orifice out of which the fiber stream passes. The absence in the invention of disruption external to the orifice is essential to the formation of a web in which the fibers are continuous and distinctly oriented in a controlled fashion.
Thus the fibers of the present invention are generated, controlled, and collected in a manner which produces a web of oriented fibers, contrasted with an heterogeneous intertwined arrangement for making fibers, that has no control over the path of the fibers.
While finer fibers and improved collection are made possible in existing systems by the use of individual nozzles, their use has the disadvantage that the product web has a stripy appearance. The stripes reflect the spacing between adjacent nozzles.
The present invention provides for ameliorating at least some of the disadvantages of the prior art melt-blown fibrous webs and the methods for producing them.
The present invention provides a convenient means to collect the output of individual fiberizing nozzles in the form of a web which is not only substantially free of striping, but is characterized by a high degree of uniformity, for example by weight distribution varying less than about 1% over a fifty cm span. Such a degree of uniformity makes the product useful in applications such as diagnostic devices, as well as for other applications where near to perfect uniformity is required. The products of the present invention are substantially free of shot and roping.
Diagnostic Devices
As indicated above, the present invention provides a uniform product that is especially desirable for use in diagnostic devices.
Many body fluid processing protocols, particularly those involving diagnostic testing, include determining whether a particular substance, e.g., a target analyte, is present in the body fluid. Many of these tests rely on calorimetric or spectrophotometric evaluation of a reaction of a fluid component with one or more specific reagents. Other tests include, for example, evaluating changes in pH or electrical conductance to determine the presence of the analyte. However, these tests may yield less than optimum results, since, for example, the fluid may fail to efficiently wet the test device, and/or other substances present in the body fluid may interfere with the particular substance to be analyzed and/or cause difficulties in interpreting the test results.
Illustratively, when the body fluid to be tested is blood, the red color due to the presence of red blood cells and/or the hemoglobin released by hemolyzed red cells may interfere with diagnostic tests which employ color change as part of their procedure. Accordingly, many body fluid testing protocols include separating one or more components from the body fluid before testing. For example, plasma or serum may be separated from blood before subjecting the plasma or serum to analysis, so that cellular material, e.g., red and/or white blood cells, will not interfere with the test results.
One technique for separating plasma or serum from other blood components, for example, cellular components such as red and white blood cells, includes obtaining blood, e.g., from a finger prick, and placing the blood on a blood test strip. The test strip, which includes at least one porous element, allows blood to flow into the strip, and a portion of plasma to be separated from the cells contained in the blood sample. Some test strips may include a plurality of porous elements that allow the passage of plasma or serum therethrough, wherein at least one element may include one or more reagents that react with the analyte so that the presence of the analyte in the plasma or serum may be determined.
However, the prior art test strips suffer from a number of drawbacks. A particular drawback is a lack of product reproducibility, as the strips are difficult to produce with a sufficient degree of uniformity. For example, some strips are insufficiently uniform to provide for efficient and/or reproducible plasma separation. Illustratively, some strips include fibrous webs having a stripy appearance resulting from a lack of uniform fiber distribution, e.g., ridges of fibers. In order to minimize the effects of non-uniformity, some test strips include multiple layers of webs, e.g., about layers or more, to provide for reliable separation. In view of the number of layers, such devices may require a relatively large amount of blood to provide sufficient plasma for a diagnostic test.
Other devices, with or without fibrous media, fail to provide a sufficiently large plasma front ahead of the front of cellular material to allow testing of the plasma without interference from the cellular material, consequently, the failure of these devices to efficiently separate plasma may require the use of a relatively large blood sample to assure that sufficient plasma is available to be tested.
Additionally, since some devices include one or more reagents preplaced in one or more areas of the device a lack of product reproducibility from one device to another may lead to the failure of the plasma to contact the reagent(s) in a particular location and/or to contact the reagent(s) for a sufficient amount of time. For example, since some preplaced reagents are soluble, devices that allow the plasma to pass through too quickly may fail to allow the plasma to dissolve the reagent, leading to an inaccurate test result. Accordingly, due to a lack of uniformity, two devices may provide different test results for the same patient using consecutive drops of blood, and it may be difficult to determine which, if either, of the devices have provided an accurate result.
Furthermore, particularly for some of those strips including at least two porous elements secured to each other, it may be awkward and/or difficult to bond the elements together. Not only is the bond obtained by simply compressing two or more layers or elements together generally weak, but the layers tend to be undesirably compressed when they are pressed together to form the bond, which in turn decreases the effectiveness of plasma separation. Bonding using commercial "sticky" adhesives adversely affects flow from a layer to its neighbor(s); thus, the permeability of the bond, or the area near the bond, may be adversely affected.
Accordingly, there is an ongoing need in the art for body fluid processing devices and methods for using them that provide for efficient separation of at least one desired component of the body fluid in sufficient amounts for analysis. Such processing devices are preferred to be easy to use, whether it is by patients, or by medical personnel such as physicians, nurses, or lab technicians. Moreover, the devices should perform separation in a manner such that the test results are accurate and reproducible.
Additionally, the devices are preferred to allow efficient separation of plasma from blood without removing a significant proportion of the substance(s) or material(s) in the plasma to be analyzed or determined, e.g., glucose, cholesterol, lipids, serum enzymes, nucleic acids, viruses, bacteria, and/or coagulation factors, to name but a few.
The present invention provides for ameliorating at least some of the disadvantages of the prior art test strips and methods for using them. The present invention can also be used for protocols involving the processing of non-biological fluids. These and other advantages of the present invention will be apparent from the description as set forth below.